Mano is spot on... just to add some more.
RTAS stands for Real Time Audio Suite and refers to software effects that are host based (i.e. they are 'plugged into' your computer rather than needing extra Digidesign Hardware like the TDM effects). Like the plugins in Cubase and so on they can be used in real-time and depend on your computers processing power to function. The RT is added because pro-tools also features 'Audio Suite' effects that are done 'offline'. That is you take a chunk of Audio and select what effect you want applied and instead of being able to hear it the actual audio file is altered and then placed back in your song.
TDM is as Mano says a protocol (it stands for Time Division Multiplexing... which sounds cool). Often plugins in pro-tools are referred to as TDM because they use the DSPs (see Mano's definition) that come as part of the pro-tools TDM hardware.
KasioRoks